i think that you require both - suprised that you don't already have a DO meter for wort aeration checks and filtration purposes. might be playing with fire quality-wise. think you had better find a way to purchase both.
if you check only head space air, you may find that they are OK, but you don't know if you are sending high DO beer to the bottle in the first place.
if you check only DO, that may be great, but you may be oxidising the beer due to high air in package.
hmmm, maybe someone else has a way use only one instrument for both purposes?????